Amplifier control



Dec. 31, 1935. F. RIEBER 2,025,775

AMPLIFIER CONTROL Filed July 3, 1929 MFA/HER AMPL/F/fR.

fvPur CME B Y 4 ATTORNEY Patented Dec. 31, 1935 UNITED STATES PATENTOFFICE AMPLIFIER CONTROL 1 Frank Rieber, San Francisco, Califg assignor,by

mesne assignments, to Sound Laboratory Corporation, Ltd., a corporationof Nevada Application July 3,1929, serial No'..375,871

8 Claims. (Cl. 179 -171) This invention relates to an amplifier system,and-especially to an automatic control thereof whereby the degree ofamplification 'is maintained below an undesired value.

5 Amplifier systems are now in quite general use for a variety ofpurposes. They can be used in transmitting and receiving signals eitherby wire or radio; and such signals can be telegraphic or speech signals.Furthermore, the systems can 10 be used for simple audio frequencysignals, or

for amplifying the carrier frequency impulses In all such cases, it isadvantageous to ensure against too great a response of the apparatus'tothe signals; for otherwise serious injury may be done to the delicatelyadjusted apparatus; or else extremely great distortion of thesignal mayresult. As an instance where such great response would be annoying andharmful, attention may I be called to the photographic recording ofsound for talking pictures. When the sound reaching the microphone isvery loud, the light valve used in the recording system may bedestroyed; and

even for moderately loud signals, the systemserves to distort them.

effects by manual controls of potentiometers or other forms of controlequipment. However, even the most attentive and expert operator is yetsubject to the usual physiological response delay. Furthermore, suchclose attention and care is required that the operator cannot maintain auniformity of proficiency.

It is one of the objects of my invention, therefore, to make it possibleto regulate the degree of response in a manner-independent of manualoperation, and yet without distortion of the signals.

My control system can be easily adapted to work with the common types ofelectronic emission amplifiers. Such amplifiers usually include anevacuated vessel enclosing a plurality of electrodes. One of theelectrodes is an electron emitting electrode, or cathode, and isusually, but not necessarily, in the form of a filament heated 50 by anelectric current. The electrons thus emitted are received on an anodealso enclosed in the vessel and in the form of a plate. This anode ismaintained at a potential positive with respect to the filament, as bythe aid of a source of potential located in a circuit externallyconnecting It has been proposed to guardagainst these vention;- but 1 itis to be understood that this dethe'two electrodes. The positivepotential impressed on the plate serves to cause it to attract theelectrons, which fiow from the cathode and constitute the "space curren2 It has been found that the space current can 5 bevaried by largeamounts by varying the po-' tential diiieren'ce, even by small'amounts,be-' tween the cathode anda point in the space between the twoelectrodes. I This variation in potential difference can be accomplishedby the aid of a control electrode interposed between the otherelectrode, and usually in theform of a grid. Weak signal impulses arecaused'to produce variations in potential between the cathode and thecontrol electrode, and cause corresponding large variations in the spacecurrent. It-in this way that an amplifier effect is secured. Theamplifled variations present in the"output circuit" (that is, theexternal circuit between the cathode and anode), can, if desired, bereamplified, bythe provision of succeeding amplifier stagea' Since suchelectron emission' devices are nowwell understood, there is no need foram further detailed description of'operation. Itmay, how-" ever, be!-mentioned that it is also common to feed back" some of the energy inthe output cu: cult to the input circuit to produce reamplificationbythe aid of the same tube, and this processis usually termed*regeneratio I 1 It is another object of my invention.- to provide anautomatic amplification limiter that can be used with electronemissionamplifiers. 5

It is still another'object of'myinvention to make it possible to limitthe degree of regenera tion obtainable by the aid of an-electronicemission amplifier; and in fact-to make it possible automatically toreterse the regenerative efiect; when theamplification tends to becomestill greater. Thus a stabilizing-' result is provided that ensuresagainst the oscillations. y a

My invention possesses-many other advantages, and has other objectswhich maybe made more easily apparent from a consideration ofone-embodiment of my invention. For this purpose I have shown a orm inthe drawing accompanying I and forming art of the present specification.I

shall now-proceed to describe this form in detail which illustrates thegeneral principles of my indetailed description is not to be taken in alimiting sense, sincethe scope of my inventionis best de-" production ofparasitic "control device or stabilizer incorporating my in- 'vention;

the signals in the amplifier output.

plication of my invention, and especially to-a-regenerative type ofelectron emission amplifier;

and

Fig. 3 is a graph illustrating the operation of the system shown in Fig.2.

In Fig. 1 I show amplifier II in diagrammatic form, which is intended tooperate on signal impulses and to increase the intens'itiesthereof: forexample, to amplify sounds reaching a-microphone 24. I also show theoutgoing circuit -I2'I'3, carrying the amplified signaling impulses andcapable of impressing said amplified impulses onto a succeedingamplifier or onto a translating device 23, such as a sound recorder, orloud speaker, or the like.

Interposed between amplifier II and circuit |2I3, is the amplificationcontrol device I4. This is in the form of a four armed Wheatstonebridge. The amplifier output connects across one diagonal thereof, andcircuit. |2|3 across the other. The bridge I4 is shown as formed of aseries of four impedances, such as resistors I5, I5, I1, I I. If theratio of impedance I5 to impedance I5 is the same as that of impedanceI8 to impedance II, it can readily be shownthat the bridge is balanced;that-is, that any potential diiference impressed by the amplifiersystem.

across points I9, 20 will cause no potential differ ence to existbetween points 2|, 22, to which the circuit I2, I3 connects. Thus undersuch circumstances, no matter how great the amplification secured by theamplifier I I, this amplfied-signal is not made available in circuitI2-I3 However, if the prescribed ratios change, the bridge becomesunbalanced, and as the ratios departmore and more from thebalancedcondition,

a greater eirec't is produced in circuit I2--I3. The device I4 is soconstructed and arranged that these ratios change. automatically andcontinuously in response to variations inthe intensity of How thiseflect is secured willnow be described.

Two opposite resistance arms such-as |6, I8, of the bridge I 4 are madefrom material such that any increase in temperature causes a very largeincrease in resistance. Such material may be iron wire, or manganin, orthe like. These-arms IS, I 8, can be enveloped in tubes to conserve heatgenerated by the flow of current therethrough, so that the greater thecurrent flow, the greater the temperature becomes, and consequently, the

greater the resistance. The other two arms I5, I! can be made fromordinary conducting wire.

It is apparent that the resistance can be so chosen that anydesiredcontrolling effect is secured. It the resistances I6, I8, aremade from fine wire, they can respond more quickly to the variations inpotential across points I9, 20 and the circuit |2-I3 is correspondinglycaused to respond quickly. On the other hand, the use of larger wire forthese resistances causes a heat lag that may produce substantial lag inresponse in the circuit I2-I3. This speed of response can also bemodified by compassing the resistances I6, I8 in material of any desiredheat conserving or insulating quality, to control the rate of heatdissipation from these resistances.

The various resistances are of course so designed that during normaloperation of the system (that is, while no unusually loud signals aretransmitted by amplifier II), the bridge is unbalanced, and an E. M. F.exists across points 2|, '22. -As'thefiiitensity0f the signal increases,resistances 16,18 increase, and this increase :serves to *bring thebridge more "nearly to balanced condition. Thereis a correspondingreductionin 'theratio between the E. M. F. across 5 points 2I,-2-2,"and'that across points I9, 2II. Thus even a very greatly increased signalefiect serves only slightly tofincrease the E. M. F. across points 2|,22. In factthe constants of the design can be chosen actually to reducethe E. M. F. across lo these-points upon too great an increase in thesignal intensity in the output side of amplifier "I I. The limit'towhich such 'a signal reduction can be carrie'djis of course alsodetermined by the radiation characteristic'of the resistors I8, I8.

In Fig. 2 I show my invention used in conjunc- 'tion with an'electronemission amplifier 25 which is connected up" to provide a feed-backeffect.

"In this 'case,;I show a conventional microphone circuitincluding amicrophone 26,-battery 21, and primary winding 28,0f a transformer 29.This circuit is shown merely asan example of one way in which signalingimpulses are passed to the amplifier tube 25.

When the microphone 26 is caused to operate 26 in response to soundsreaching it, the resistance of circuit26-2l-28 varies in accordance withthe sound waves, the currentvaries correspondiriglyfand an F. isinducedacross secondary coil. '30.

ondary '30 connects through a biasing battery 3| to the cathode $32} andthe upper end connects through a variable potentiometer resistance 33to35 the controlelectrode 34. The. potentiometer resistance can.bevaried by movement of a variable contact device 35. The output circuitof amplifier includesiplate or anode 36, primary coil 31 oi atranstormer 38, B battery 39, and filament-.40 or cathode 22. I

The variations in' space current of amplifier 25, due to variations inE. M. F. appliedto the input of amplifier 25cause corresponding variations in E. M.'F.'in the secondary coil 40 of transformer 33. Thesevarying E. M. F.'s can be impressed upon succeeding stages ofamplifiers,

such as indicated at 4|, and finally upon a translating device 142, suchas a recorder or loud speaker. The'secondary coil 4|) for thispurpose'co is connected by conductors 43, 44 to the amplifier,4|.

The E. M. F. impressed upon the input side of amplifier 25 is to someextent dependent upon the output E. MLF. thereof. Thus for weak sig- 55,

nals, the E. M. F. impressed across the input circuit is increased bythe system I have evolved; and for strong signals, the E. M. F. isreduced thereby. There is, therefore, a stabilizing effect, tending tomaintain the intensity of the output E. M. F. substantially constant.

To secure this result, a Wheatstone bridge device 45 is included, of thesame characteristics as device I4 of Fig. 1. The arms 41, 48 have a hightemperature coefllcient so that their resistances increase rapidly withincrease in current fiow, while the other two arms, 49, 50 are made fromthe usual kind of resistance wire. The output circuit of amplifier-25 iscaused to affect the E. M. F. across the diagonal points 5|, 52, as byconnecting thesepoints respectively to conductors 43, 44. p

When the ratio of resistance 49 to resistance .48 is the same as that ofresistance 41 to resistance 50, then the E. M. F. across the otherdiagonal points 53, 54, is reduced to zero; if the resistances 41, 48are less than required by these ratios, the points 53, 54 have apotential difi'erence; and if the resistances 41, 48 are greater thanrequired by these ratios, then the points 53, 54 have a potentialdifference of opposite sign. Thus this potential difierence can be usedto produce automatically either a regenerative or a reverse feed-backefi'ect, depending upon the strength of the signal.

To secure this result, points 53, 54 can be connected to afiect thepotential drop across potentio'meter resistance 33, any desired portionof which can be included in the input circuit of amplifier 25. Thuspoints 53, 54 can be connected to the primary coil 55 of a transformer56, the secondary coil 51 of which is' connected across resistance 33.It is apparent that by varying the position of tap 35 on resistance 33,any desired portion of the potential drop in resistance 33 can beincluded in the input circuit. The direction of this drop is chosen tobe in phase with the E. M. F. across coil 30, when the signals are weak;and of course they are automatically caused to be out of phase when thesignals are strong, due to the automatic selection of the E. M. F.across points 53, 54 as just explained.

Athough rapidly varying currents flow through the circuits, yet theresistances 41, 48 can. be readily caused to respond to them. p

The controlling effect of the variable tap 35 is best illustrated inconnection with Fig. 3. The ordinates of curves A, B, C represent outputE. M. R's across points 52. The abscissae represent input E. M. F.s, saythat existing across coil 3!). Curve A shows a rising character-- isticin general, the greater the input E. M. F. the greater the output E. M.F. Curve B shows a flat characteristic over a wide range, indicating novariation of output asthe input E. M. F. varies; while curve C shows acharacteristic that first rises to a maximum and then declines uponfurther increase in the inputE. M. F. All these characteristics can beobtained by varying the position of contact 35.

It is apparent that I have provided a system that serves to prevent theproduction of undesirably intense signals, and yet without any danger ofproducing distortion.

I claim:

1. In an amplifier system for electric signal impulses, an amplifier, aWheatstone bridge having resistance arms, means for impressing all thesignaling impulses from the amplifier across a diagonal of the bridge,and an outgoing circuit connected across the other diagonal, theresistance of at least one of the arms varying directly with the currentresulting from the intensity of the impressed signaling impulses,whereby the potential difference across said other diagonal iscontrolled in accordance with the said intensity.

2. In an amplifier system for electric signal impulses, an amplifier, aWheatstone bridge having resistance arms, a pair of opposite resistancearms having a characteristic such that variations in current fiowtherein cause substantial variations in resistance in said arms, meansfor impressing all the signaling impulses from the amplifier across adiagonal of thebridge, and an outgoing circuit connected across theother diagonal.

3. The combination as set forth in claim 2, in

which the resistance characteristic of said pair 5 of opposite arms areso chosen that as the signal intensity increases, the ratio of theelectromotive force passed by the bridge to the outgoing circuit, to thesignaling electromotive force is reduced.

4. In a feed-back amplifier system for signaling impulses, an amplifier,an input and an output circuit therefor, a Wheatstone bridge having fourresistance arms, one diagonal of the bridge being connected so as to beafiected by the electromotive force in the output circuit,

and the other diagonal being connected to a circuit coupled to the inputcircuit so as to affect the input circuit electromotive force at leastone of the arms being constructed so that an increase in current flowtherein causes a substantially large increase in resistance.

' 5. The combination as set forth in claim 4, in which the arms of thebridge are so proportioned that for weak signals the electromotive forceacross the said other diagonal serves to whereby the potentiometer mayvariably affect the input circuit, at least one of the arms beingconstructed so that an increase in current fiow therein causes asubstantially large increase in, resistance.

7. The combination as set forth in claim 6, in

, which the arms of the bridge are so proportioned that for weak signalstheelectromotive force across the said other diagonal serves to producepositive feed-back, and for strong signals, negative feed-back.

8. In a feed-back amplifier system for'signab ing impulses, anamplifier, an input and an output circuit therefor, and means forinterconnecting the input and output circuits to secure a feed-back,said means including a Wheatstone bridge across one diagonal of whichthe output circuit is connected, and a coupling circuit connected acrossthe other diagonal of the bridge .56 and inductively coupled to theinput circuit, at least one of the impedances of the bridge in creasingin value with an increased current flow therethrough, the bridgeimpedances cooperating to cause a reversal of potential across theseconxl 00 diagonal with respect to the first diagonal when a potentialdifference exceeding a predetermined maximum potential difference isimpressed across the first diagonal, whereby the energy fed by thecoupling circuit to the input circuit reo5 verses in direction when theenergy in the output circuit increases beyond a definite value.

FRANK RIEBER.

